The present invention relates generally to gas turbine engines, and, more specifically, to turbofan engines.
A turbofan gas turbine engine includes a fan for pressurizing ambient air to produce propulsion thrust for powering an aircraft in flight, with the fan being powered by a core engine. Disposed downstream from the fan is a multi-stage axial compressor that pressurizes a portion of the fan air which is then mixed with fuel in a combustor for generating hot combustion gases which flow downstream through turbine stages that extract energy therefrom. A high pressure turbine powers the compressor by rotating a shaft therebetween. And, a low pressure turbine powers the fan by rotating a fan shaft therebetween.
The fan shaft is supported in a bearing near the fan with the bearing in turn being supported by a bearing support fixedly joined to a stationary fan frame. During normal operation, the fan rotates dynamically balanced, and the fan bearing maintains concentric alignment of the fan within a surrounding fan casing, and carries operational loads into the fan frame.
The fan includes a row of large rotor fan blades extending radially outwardly from a supporting rotor disk, and is subject to foreign object damage (FOD). For example, a large bird may be ingested by the engine and strike one or more of the fan blades causing substantial damage thereto including liberation thereof from the supporting fan disk. Accordingly, a substantial rotary imbalance load will be created in the damaged fan, which imbalance load must be suitably carried by the fan bearing, its support, and fan frame.
In order to accommodate the possibility of such a large abnormal imbalance load, the various supporting components for the fan may be sized for additional strength required therefor. However, the stronger supporting components undesirably increase weight of the engine and decrease overall efficiency of the engine when used in normal operation without substantial rotor imbalance.
Another solution for large imbalance loads is the introduction of a bearing support which intentionally severs in the imbalance event for decoupling the fan from the bearing support. In this event, the fan is supported by its relatively flexible fan shaft which reduces the fan critical speed well below the maximum operating speed thereof. The fan accordingly operates dynamically supercritical for significantly reducing orbit of the fan disk and imbalance loads therefrom. The fan speed is then reduced and crosses the fan critical speed at a relatively low value with rapid deceleration having correspondingly reduced peak loads therefrom.
The stiffened bearing support configuration is sufficiently strong to prevent any structural failure thereof. However, by softening the structural loadpath to introduce intentional failure for abnormal fan loads, the loadpath components are subject to undesirable fatigue damage which could adversely reduce the life thereof.
The ability to introduce the decoupling bearing support is limited by the particular bearing support design and available space. Since the bearing support is located radially inwardly of the fan blades, little available space is provided for introducing decoupling features without undesirably increasing the overall diameter of the fan. And, the decoupling configuration should have minimal variability between the maximum load capability thereof prior to the decoupling failure and the minimum load capability for normal operation without accumulating life-limiting fatigue damage.
Accordingly, it is desired to provide an improved fan decoupling system with minimal variability between maximum load capability and minimum load capability without life-limiting fatigue damage.
A fan decoupling fuse includes a ring having a row of fuse holes circumferentially spaced apart from each other by fuse ligaments sized to fail under shear when carrying abnormal radial loads from the fan.